The present invention relates to improvements in a shift control apparatus of a continuously variable transmission (CVT), and more particularly to a shift control apparatus arranged to accurately decide a shift mode for determining a shift control dynamic characteristic which is employed to realize a final objective transmission ratio according to a running condition of a vehicle.
A Japanese Patent Provisional Publication No. 8-74958 discloses a typical shift control apparatus of a continuously variable transmission (CVT) which apparatus is arranged to compare a basic transmission ratio determined according to an operating condition of a CVT equipped vehicle with a transient controlled objective transmission ratio, in order to determine whether the selected shift is an upshift or an downshift. Further, the conventional shift control apparatus is arranged to determine whether the executed upshift is an automatic upshift or a power-down upshift, by checking a rate of change in a throttle opening TVO of an engine of a vehicle. Herein, the automatic upshift means an upshift executed automatically according to the increase of the vehicle speed, and the power-down upshift means an upshift executed at a time when a foot of a driver is released from an accelerator pedal of the engine. On the basis of the determined shift mode, the conventional shift control apparatus determines a suitable shift speed (a rate of change in the transmission ratio) of the CVT by each of the downshift, the power-down upshift and the automatic upshift, and calculates a transient controlled objective transmission ratio for changing the real transmission ratio to the basic transmission ratio by the determined shift speed.
However, the conventional shift control apparatus has a possibility of incorrectly determining an automatic upshift as the power-down upshift since arranged to determine that the power-down upshift is executed when the rate of change in the throttle opening TVO is greater than a predetermined value or when an absolute value of the rate of change in the basic transmission ratio is greater than a predetermined value. The operation of the conventional shift control apparatus will be mentioned as to a case that an accelerator pedal is depressed so as to generate a time series change of the throttle opening with reference to time charts of FIGS. 8A to 8E. During a period between moments t.sub.1 and t.sub.2 in FIGS. 8A to 8E, a rate .vertline..DELTA.ATVO.vertline. of change in the throttle opening TVO per time is greater than a predetermined value .DELTA.TVO(S), or a rate .vertline..DELTA.i.sub.PO .vertline. of change in the basic transmission ratio i.sub.PO per time is greater than a predetermined value .DELTA.i.sub.PO (S). Therefore, the conventional shift control apparatus correctly determines the shift mode as the power-down upshift. However, during a period between moments t.sub.2 and t.sub.3, the rate .vertline..DELTA.TVO.vertline. of change in the throttle opening TVO is smaller than a predetermined value .DELTA.TVO(S), and the rate .vertline..DELTA.i.sub.PO .vertline. of change in the basic transmission ratio i.sub.PO is smaller than a predetermined value .DELTA.i.sub.PO (S). Therefore, the conventional shift control apparatus makes an incorrect decision that the shift mode during the period between the moments t.sub.2 and t.sub.3 is an automatic upshift although the power-down upshift should be still maintained. In such a case, the transient controlled objective transmission ratio i.sub.PT is determined so as to select a shift speed for the automatic upshift on the basis of the incorrect decision as shown by an alternate long and short dash line .alpha.(i.sub.PT) of FIG. 8C. That is, although a driver is requiring a slow shift speed corresponding to that of the power-down upshift such that the transient controlled objective transmission ratio i.sub.PT changes on a dotted line .beta.(i.sub.PT) of FIG. 8C, the conventional shift control apparatus selects a quick shift speed which provides a strange feeling to the driver.